THE MORPHOLOGY OF FERTILIZATION 



a 



to become vesicular. This change, however, proceeds 

 relatively slowly during the maturation of the egg; 

 in some cases, in which the enlargement of the sperm 

 nucleus begins in the early stages of maturation of the 

 egg, there is a halt or actual decrease in size during the 

 later stages until the second polar body is formed, 

 when the definitive increase in size ensues (e.g., Unio, 

 according to Lillie, 1895). 

 In such cases the sperm 

 nucleus after penetrating 

 a short distance may cease 

 to move (Unio), or it may 

 penetrate to the center of 

 the egg and then come 

 to rest (Nereis, Fig. 8c). 

 After maturation is com- 

 pleted the sperm nucleus 

 and the egg nucleus en- 

 large in practically equal 

 tempo and come together 

 in a predetermined region 

 of the egg (Figs. 8d, e, 9). 

 They are then usually of the same size and appearance, 

 so that they can be distinguished only by their positions 

 or associations. 



When maturation of the egg is completed before 

 fertilization, as in the sea urchin, the egg nucleus and 

 the sperm nucleus proceed directly to the place of 

 meeting, and the sperm nucleus is much smaller than the 

 egg nucleus at the time of union (Fig. 7 g, h, i). The 

 chromatin is, however, much more condensed and is 

 equal in quantity to that of the egg nucleus; if the nuclei 





FIG. 8 (continued). e, Somewhat 

 later. The two germ nuclei have 

 fused and the first cleavage spindle 

 is forming with unequal centers. 

 The chromosomes of sperm origin 

 lie below; those of egg origin, above. 

 Only a few of the latter fell in the 

 plane of section. 



